The disclosure of Japanese Patent Application Nos. 2000-00434 filed on Mar. 29, 2000 and 2000-04433 filed on Nov. 10, 2000 including the specification, drawings and abstract are incorporated herein by reference in their entirety.
1. Field of the Invention
The invention relates to an exhaust purifying method and apparatus of an internal combustion engine. More particularly, the invention relates to an exhaust purifying method and apparatus capable of switching an exhaust gas flow such that the exhaust gas flows through a filter of the purifying apparatus alternately from the exhaust upstream side and exhaust downstream side.
2. Description of Related Art
In diesel engines, a particulate filter is provided in an exhaust passage of the engine in order to remove particulates such as soot contained in the exhaust gas. The particulates in the exhaust gas are temporarily trapped on the particulate filter and then ignited and burned so as to restore the particulate filter. However, the particulates trapped on the particulate filter are not ignited at a high temperature of about 600xc2x0 C. or more, although the exhaust gas temperature of the diesel engine is normally considerably lower than 600xc2x0 C. Accordingly, it is difficult to ignite the particulates trapped on the particulate filter with the exhaust gas heat. In order to ignite the particulates trapped on the particulate filter with the exhaust gas heat, it is necessary to enable the particulates to be ignited at a low temperature.
It is known that a particulate filter having a catalyst supported thereon can reduce the ignition temperature of the particulates. A variety of particulate filters are known which have a catalyst supported thereon in order to reduce the ignition temperature of the particulates.
For example, Japanese Patent Publication No. HEI 7-106290 discloses a particulate filter having a mixture of a platinum group metal and an alkaline earth metal oxide supported thereon. In this particulate filter, particulates are ignited at a relatively low temperature of approximately 350xc2x0 C. to 400xc2x0 C., and then burned continuously.
In diesel engines, the exhaust gas temperature reaches 350xc2x0 C. to 400xc2x0 C. under a high load. Accordingly, in the aforementioned particulate filter, the particulates can seemingly be ignited and burned with the exhaust gas heat under a high load. Actually, however, the particulates are not always ignited even when the exhaust gas temperature reaches 350xc2x0 C. to 400xc2x0 C. Moreover, even if the particulates are ignited, only a part of the particulates is burned, and a large amount of particulates is left unburned.
In other words, if the exhaust gas contains a small amount of particulates, the amount of particulates that adhere to the particulate filter is small. In such a case, the particulates on the particulate filter are ignited when the exhaust gas temperature reaches 350xc2x0 C. to 400xc2x0 C., and then burned continuously.
However, if the exhaust gas contains a large amount of particulates, additional particulates are deposited on the particulates adhering to the particulate filter before the latter particulates have been burned completely. As a result, the particulates are deposited on the particulate filter in a laminated manner. In such a case, a part of the particulates that is likely to be in contact with oxygen is burned, but the remaining particulates that are less likely to be in contact with oxygen are not burned, and a large amount of particulates are left unburned. Therefore, when the exhaust gas contains a large amount of particulates, a large amount of particulates continues to be deposited on the particulate filter.
If a large amount of particulates is deposited on the particulate filter, these deposited particulates gradually become less likely to be ignited and burned. The reason for this is considered because, while the particulates are being deposited, carbon in the particulates changes to a substance that is less likely to be burned, such as graphite or the like. Actually, if a large amount of particulates is continuously deposited on the particulate filter, the deposited particulates are not ignited at a low temperature of 350xc2x0 C. to 400xc2x0 C. A high temperature of 600xc2x0 C. or more is required in order to ignite the deposited particulates. In diesel engines, however, the exhaust gas temperature does not normally reach a high temperature of 600xc2x0 C. or more. Accordingly, if a large amount of particulates is continuously deposited on the particulate filter, it becomes difficult to ignite the deposited particulates with the exhaust gas heat.
Moreover, when the deposited particles are burned, ashes, i.e., the matter left after the particulates have been burned, agglomerate into a large mass, resulting in clogging of the pores in the particulate filter. The number of clogged pores is gradually increased with time, so that pressure loss of the exhaust gas flow in the particulate filter is gradually increased As the pressure loss of the exhaust gas flow is increased, the engine output is reduced, which also necessitates early replacement of the particulate filter with a new one.
Thus, once a large amount of particulates has been deposited in a laminated manner, this causes various problems as described above. Therefore, it is necessary to prevent a large amount of particulates from being deposited in a laminated manner, in view of the balance between the amount of particulates contained in the exhaust gas and the amount of particulates that can be burned on the particulate filter.
The aforementioned problems cannot be avoided by such a continuous combustion process that a conventional exhaust purifying filter having a catalyst thereon is merely provided in an exhaust pipe and that exhaust purification relies on the operation state of the internal combustion engine.
Therefore, by enabling an exhaust gas flow to be switched such that the exhaust gas flows through a filter of a purifying apparatus alternately from the exhaust upstream side and downstream side in order to enable continuous combustion of the particulates as much as possible, the particulates are deposited on both sides of the filter, whereby the particulate deposition amount per unit area can be reduced. Moreover, switching of the exhaust gas flow enables the depositing particles to be stirred and scattered. Furthermore, if a NOx absorbent is provided for a filter base material, NOx in the exhaust gas can also be purified.
In the case where the NOx absorbent is provided for the filter base material in order to simultaneously purify NOx, exhaust gas at a rich air-fuel ratio must be intermittently fed to the filter (this is called xe2x80x9crich spikexe2x80x9d) so as to discharge NOx from the NOx absorbent for reduction. This is because of limited NOx-absorbing ability of the NOx absorbent. On the other hand, in order to switch the direction of the exhaust gas flowing through the filter as described above for the purpose of facilitating continuous combustion of the particulates, a switch valve must be provided within the exhaust pipe. However, the structure of the switch valve necessitates the exhaust gas to bypass the filter during switching of the exhaust gas flow.
Accordingly, in the case where the timing of conducting the rich spike matches the timing of switching the exhaust gas flow by the switch valve, the exhaust gas at a rich air-fuel ratio and containing a large amount of reducing agent may possibly be discharged without passing through the filter.
In the case of using the exhaust purifying filter having the NOx absorbent at the filter base material in order to simultaneously remove the particulates and purify NOx, a system for feeding the exhaust gas so as to bypass the filter is possible in order to prevent the particulates from being deposited on the filter in a preset amount or more when the particulate oxidation capability of the filter is not enough, or may possibly be degraded due to a low temperature of the exhaust gas (e.g., during decelerating operation).
Even in such a system, if the rich spike for purifying NOx is conducted while the exhaust gas is being fed so as to bypass the filter, the exhaust gas at a rich air-fuel ratio and containing a large amount of reducing agent is discharged without passing through the filter.
The invention is made in view of the above, and it is an object of the invention to prevent, in an exhaust purifying method and apparatus provided with a filter having a NOx absorbent, exhaust gas containing a large amount of reducing agent required to discharge NOx from the NOx absorbent from being discharged untreated.
It is another object of the invention to provide an exhaust purifying method and apparatus of an internal combustion engine capable of preventing, while unburned fuel is being supplied to a filter in order to improve the particulate oxidation removal capability of the filter, the unburned fuel from bypassing the filter and thus being discharged to the outside of the vehicle without being supplied to the filter.
It is a further object of the invention to provide an exhaust purifying method and apparatus of an internal combustion engine capable of preventing, while exhaust gas at a relatively small air-fuel ratio and containing HC, CO, unburned fuel and the like is flowing, the exhaust gas from being discharged into the atmosphere without passing through a particulate filter.
An exhaust purifying method and apparatus of an internal combustion engine in a first aspect of the invention includes: a filter having a NOx absorbent and an active-oxygen discharging agent supported thereon, and capable of temporarily trapping particulates contained in exhaust gas, the NOx absorbent absorbing NOx when the exhaust gas flowing therein has a lean air-fuel ratio and discharging the absorbed NOx when an oxygen concentration in the exhaust gas flowing therein is reduced, the active-oxygen discharging agent facilitating oxidation of the particulates; a valve having a switching operation capable of alternately switching between a first flow for feeding the exhaust gas from one side of the filter and a second flow for feeding the exhaust gas from the other side of the filer, the exhaust gas flowing so as to bypass the filter during the switching; a controller that inhibits simultaneous control of an air-fuel ratio operation of the exhaust gas flowing into the filter to a theoretical or rich air-fuel ratio so as to discharge NOx from the NOx absorbent supported on the filter, and the switching operation of the valve.
In the first aspect, the controller inhibits the air-fuel ratio operation and the switching operation of the valve from being conducted simultaneously. Therefore, the exhaust gas is controlled to the theoretical or rich air-fuel ratio in order to discharge NOx from the NOx absorbent is prevented from being discharged untreated without passing through the filter.
In the first aspect, the switching operation of the valve can be normally conducted in every decelerating operation, at predetermined time intervals, at every predetermined running distance or the like, and is not particularly limited. In the first aspect, the valve may be composed of a switch valve capable of switching a flow direction of the exhaust gas in the filter between forward and reverse directions.
A second aspect of the invention includes: a filter for temporarily trapping particulates contained in exhaust gas and removing the particulates by oxidation; a valve having a switching operation capable of alternately switching between a first flow for feeding the exhaust gas from one side of the filter and a second flow for feeding the exhaust gas from the other side of the filer, the exhaust gas flowing so as to bypass the filter during the switching; a controller that controls supplying unburned fuel to the filter while simultaneously inhibiting the switching operation of the valve from being conducted.
In the second aspect, the unburned fuel is supplied to the filter. As a result, the unburned filter is also oxidized on the filter, so that the filter temperature rises. By switching the exhaust gas flow at this time, oxidation heat is prevented from leaving the filter, so that the filter temperature further rises. Such rise in filter temperature improves the particulate oxidation removal capability of the filter. Accordingly, it is preferable to supply the unburned fuel to the filter at a predetermined timing. However, if the unburned fuel supply and the switching operation of the valve are conducted simultaneously, the unburned fuel bypasses the filter so as to be discharged to the outside of the vehicle. By providing the controller for inhibiting the unburned fuel supply and the switching operation of the valve from being conducted simultaneously, the unburned fuel being supplied to the filter can be prevented from bypassing the filter and thus being discharged to the outside of the vehicle without being supplied to the filter.
A third aspect of the invention includes: a filter having a NOx absorbent and an active-oxygen discharging agent supported thereon, and capable of temporarily trapping particulates contained in exhaust gas, the NOx absorbent absorbing NOx when the exhaust gas flowing therein has a lean air-fuel ratio and discharging the absorbed NOx when an oxygen concentration in the exhaust gas flowing therein is reduced, the active-oxygen discharging agent facilitating oxidation of the particulates; a controller that provides simultaneous control of an air-fuel ratio of the exhaust gas flowing into the filter to a theoretical or rich air-fuel ratio so as to discharge NOx from the NOx absorbent supported on the filter; a valve that normally feeds the exhaust gas to the filter, but feeding the exhaust gas so as to bypass the filter when particulate oxidation capability of the filter is lower than, or may become lower than, a predetermined level; and a bypass operation that inhibits, even if the particulate oxidation capability of the filter is lower than the predetermined level, the valve from feeding the exhaust gas so as to bypass the filter, when the controller conducts the air-fuel ratio so as to discharge NOx from the NOx absorbent supported on the filter.
In the third aspect, in principle, the valve feeds the exhaust gas so as to bypass the filter, if the particulate oxidation capability of the filter is lower than the predetermined level. Therefore, the particulates can be prevented from being deposited on the filter in a preset amount or more. However, in the case where NOx is to be discharged from the NOx absorbent, the bypass operation inhibits the exhaust gas from bypassing the filter and thus ensures that the exhaust gas flows into the filter, even if the particulate oxidation capability of the filter is lower than the predetermined level. Therefore, the exhaust gas is controlled to the theoretical or rich air-fuel ratio in order to discharge NOx from the NOx absorbent is prevented from being discharged untreated without passing through the filter.
The third aspect may have a function to normally feed the exhaust gas to the filter, but to feed the exhaust gas so as to bypass the filter when the particulate oxidation capability of the filter is lower than, or may become lower than, a predetermined level.
In the third aspect, xe2x80x9cwhen the oxidation capability of the filter is lower than the predetermined levelxe2x80x9d may be, for example, during load-reducing operation of the engine (during vehicle decelerating operation in the case of an internal combustion engine for driving the vehicle).
A fourth aspect of the invention is an exhaust purifying method and apparatus of an internal combustion engine, wherein a particulate filter for trapping particulates contained in exhaust gas discharged from a combustion chamber is provided in an exhaust passage of the engine, so that the particulates contained in the exhaust gas are trapped when the exhaust gas passes through a wall of the particulate filter. The particulate filter is capable of oxidizing the particulates temporarily trapped thereon. The exhaust purifying method and apparatus is further provided with a valve that inverts a flow of the exhaust gas passing through the wall of the particulate filter. The valve has a bypass mode for causing the exhaust gas to bypass the particulate filter without flowing therein. While a first exhaust gas at a relatively small air-fuel ratio is flowing, the valve is inhibited from being rendered in the bypass mode. While a second exhaust gas at a relatively large air-fuel ratio is flowing, the valve is allowed to be rendered in the bypass mode.
In the fourth aspect, while the first exhaust gas at a relatively small air-fuel ratio is flowing, the valve that inverts the flow of the exhaust gas passing through the wall of the particulate filter is inhibited from being rendered in the bypass mode. In other words, while the first exhaust gas at a relatively small air-fuel ratio is flowing, the exhaust gas is inhibited from bypassing the particulate filter without flowing therein. Accordingly, while the exhaust gas at a relatively small air-fuel ratio and containing HC, CO, unburned fuel and the like is flowing, the exhaust gas can be prevented from bypassing the particulate filter and thus being discharged into the atmosphere without passing through the particulate filter.
In a fifth aspect of the invention, if the exhaust gas discharged from the combustion chamber is flowing during low-temperature combustion in which a temperature of fuel and its surrounding gas in the combustion chamber becomes lower than a soot producing temperature and the soot is hardly generated, the valve is inhibited from being rendered in the bypass mode. The low-temperature combustion is conducted when SOx poisoning is to be restored, when NOx is to be discharged, or during low and medium load operation of the engine. Therefore, in the case where the exhaust gas containing HC, CO, unburned fuel and the like that is discharged from the combustion chamber is flowing during the low-temperature combustion conducted when SOx poisoning is to be restored, when NOx is to be discharged, or during the low and medium load operation of the engine, the exhaust gas can be prevented from bypassing the particulate filter and thus being discharged into the atmosphere without passing thorough the particulate filter.
A sixth aspect of the invention is an exhaust purifying method and apparatus of an internal combustion engine, wherein a particulate filter for trapping particulates contained in the exhaust gas discharged from a combustion chamber is provided in an exhaust passage of the engine, so that the particulates contained in the exhaust gas are trapped when the exhaust gas passes through a wall of the particulate filter. The particulate filter is capable of oxidizing the particulates temporarily trapped thereon. A NOx absorbent for absorbing NOx in a lean range and discharging NOx in a stoichiometric or rich range is supported on the particulate filter. The exhaust purifying method and apparatus is further provided with a valve that inverts a flow of the exhaust gas passing through the wall of the particulate filter. The valve has a bypass mode for causing the exhaust gas to bypass the particulate filter without flowing therein. Normally, the valve is rendered in the bypass mode when particulate oxidation capability of the particulate filter is lower than a predetermined level. However, in the case where NOx is to be discharged from the NOx absorbent, the valve is inhibited from being rendered in the bypass mode, even if the particulate oxidation capability of the particulate filter is lower than the predetermined level.
In the sixth aspect, the valve is normally rendered in the bypass mode when the particulate oxidation capability of the particulate filter is lower than the predetermined level, e.g., during decelerating operation of the engine. However, in the case where NOx is to be discharged from the NOx absorbent, the valve is inhibited from being rendered in the bypass mode, even if the particulate oxidation capability of the particulate filter is lower than the predetermined level. Therefore, normally, when the particulate oxidation capability of the particulate filter is lower than the predetermined level, increase in deposition amount of the particulates on the particulate filter can be suppressed that results from the fact that the exhaust gas possibly containing particulates passes through the particulate filter. In addition, in the case where NOx is to be discharged from the NOx absorbent, the exhaust gas containing HC, CO, unburned fuel and the like in order to discharge NOx from the NOx absorbent can be prevented from bypassing the particulate filter and thus being directly discharged into the atmosphere.
The aspects of the invention are not limited to the exhaust purifying method and apparatuses of the internal combustion engine as described above. Other aspects of the invention include, for example, a vehicle provided with the exhaust purifying apparatus of the invention, and a method for purifying the exhaust gas discharged from the combustion chamber of the internal combustion engine.